Additive manufacturing has been used for many years. Fabricated parts have been produced using various printing techniques (e.g., three-dimensional or 3-D printing techniques). For example, sheet welding, wire welding, melting in powder beds or powder deposition via laser and electron beam melting, injections using powder, liquid ultraviolet curable resins, and fusible thermoplastic filaments have all been used. These techniques have varying degrees of geometric complexity, but generally have few restrictions in comparison to conventional machining. Each type of technique has associated with it advantages and disadvantages, particularly with respect to solid state processing, fine grain structures, and mechanical properties.
Selective laser sintering (SLS) is a powder-based layer additive manufacturing process in which laser beams, either continuous or pulse mode, are used as a heat source for scanning and joining powders in predetermined sizes and shapes of layers via a polymer binder. The geometry of the scanned layers corresponds to the various cross-sections of the computer-aided design (CAD) models. A drawback of SLS is that additional powder at the boundaries is often hardened and remains attached to the part, thereby requiring additional finishing steps to remove the unwanted material. Furthermore, an inert atmosphere is often required, increasing the cost of the equipment.
Other known processes of additive manufacturing are based on a fluidic or liquid resin layer that is selectively solidified to construct a part layer-by-layer. One such process is known as stereolithography (SLA). SLA uses a vat of liquid ultraviolet curable photopolymer “resin” and an ultraviolet laser to build parts' layers one at a time. The laser beam traces a cross-section of the part pattern on the surface of the liquid resin. The UV exposure cures and solidifies the pattern traced on the resin and joins it to the layer below. After the pattern has been traced, an elevator platform supporting the vat containing the resin descends a distance equal to the thickness of the single layer. A new layer of liquid resin then forms over the part to form a new liquid surface. A subsequent layer pattern is then traced, joining the previous layer. This process is repeated to form a 3-dimensional part. The completed part is washed in a chemical bath to remove excess resin. The part is then photocured in an ultraviolet oven. Although SLA can be used to prepare parts having a variety of different shapes, the ultraviolet curable photopolymer resin can be quite expensive, and due to the complexity of the SLA equipment, the machines costs may be prohibitively expensive. Further, photocuring is limited in terms of thickness of the part that can be cured due to the photon gradient parallel to the direction of the radiation source that arises from the absorption of the radiation that must occur to effect curing.
Material extrusion is another additive manufacturing process that utilizes a fluid resin to build a part layer-by-layer. In this process, a part is formed by the extrusion of small beads of a molten thermoplastic material that fuse to each other to form layers of the part. The molten thermoplastic material hardens by cooling below its melting temperature or glass transition temperature after extrusion from a mobile nozzle. Typically, the nozzle heats the thermoplastic material above its glass transition temperature, and for crystalline or semi-crystalline material, above the melting point. The molten material is then deposited by an extrusion head. Examples of thermoplastic materials that are used in material extrusion include, acrylonitrile-butadiene-styrene (ABS), polylactic acid, polycarbonate, polyamides, polystyrene, and lignin. Material extrusion was first developed by Stratays, Inc. and is also known under the trademark FUSED DEPOSITION MODELING™.
While material extrusion generally provides an effective process for building a part, it does have some disadvantages. First, prior to adding a successive layer, the previously built layer needs to sufficiently cool and solidify. Second, the part may have lower strength in the Z-direction (e.g., between successive layers) due to poor entanglement of polymer chains between successive layers. In addition, for many thermoplastics, such as ABS, it is necessary to first dry the resin prior to extrusion.
Accordingly, a need still exists for new resins and processes to be used in additive manufacturing.